A Fabry-Perot interferometer has been disclosed in U.S. Pat. No. 5,646,729 corresponding to JP 3457373 and U.S. Pat. No. 7,733,495 corresponding to JP-A-2008-134388. The Fabry-Perot interferometer has a pair of mirrors. Each mirror includes high refractive layers and a low refractive layer sandwiched between the high refractive layers. The high refractive layer is made of a semiconductor material such as silicon or germanium. The low refractive layer is made of such as air, silicon oxide, or silicon nitride. One mirror is configured as a fixed mirror, and the other mirror is configured as a movable mirror. In U.S. Pat. No. 5,646,729, the low refractive layer is a silicon dioxide layer. In U.S. Pat. No. 7,733,495, the low refractive layer is an air layer.
Impurities are doped into the high refractive layer to form an electrode. A voltage is applied between the electrodes of the mirrors so that the movable mirror can be displaced by an electrostatic force. Thus, a distance between the mirrors is adjusted to selectively transmits light of a predetermined wavelength corresponding to the distance.
A spectral band of the conventional Fabry-Perot interferometer may be increased by increasing the amount of displacement of the movable mirror.
As is well known, the electrostatic force generated upon application of the voltage to the electrodes of the mirrors is inversely proportional to the square of the distance between the electrodes. In contrast, the restoring force of the movable mirror is proportional to a change in the distance between the electrodes. In the conventional Fabry-Perot interferometer disclosed in U.S. Pat. No. 5,646,729 and U.S. Pat. No. 7,733,495, when the change in the distance exceeds one-third of the initial distance (i.e., the amount of displacement of the movable mirror exceeds one-third of the initial distance), the electrostatic force exceeds the restoring force so that the movable mirror can stick to the fixed mirror. This phenomenon is known as a pull-in. Once a pull-in occurs, the movable mirror cannot return to its original position even after the voltage is removed. Therefore, it is difficult to increase the spectral band of the conventional Fabry-Perot interferometer by increasing the amount of displacement of the movable mirror.
JP-A-2010-8644 and JP-A-2008-517784 corresponding to US 2008/0135385 disclose a technique for preventing the pull-in. The technique disclosed in JP-A-2010-8644 and US 2008/0135385 may be applied to the conventional Fabry-Perot interferometer disclosed in U.S. Pat. No. 5,646,729 and U.S. Pat. No. 7,733,495 to prevent the pull-in. However, even if the technique is applied to the conventional Fabry-Perot interferometer, the amount of displacement of the movable mirror cannot exceed one-third of the initial distance. Therefore, the spectral band cannot be increased.